Interferons (IFNs) are critical mediators of host defense, and although they are used clinically to treat a number of diseases, their precise mechanisms of action remain to be completely understood. IFNs induce the transcription of a subset of genes called IFN stimulated genes (ISGs) that are responsible for complex biological effects. Only by understanding the function of individual ISGs will the molecular basis of IFN antiviral activity be realized. Expression of one ISG, ISG54/IFIT2, promotes apoptosis, and the broad objectives of this proposal are to determine the protective role of ISG54 in innate immunity and in viral latency. Mice have been generated with a targeted deletion in the ISG54 gene, and these mice have increased susceptibility to latency establishment by murine -herpesvirus 68. Host defense mechanisms are uniquely challenged by herpesviruses which evade innate and adaptive immune controls to establish chronic, life-long infections. There is a need to elucidate the dynamic interface between virus and host in order to harness the action of host defense mechanisms in the development of novel therapeutics. The -herpesviruses, Epstein Barr virus and Kaposi's sarcoma-associated herpesvirus, are associated with morbidity and mortality worldwide resulting from lymphoproliferative disease, lymphoma, carcinoma, and sarcoma, especially in immunosuppressed individuals. Therapeutic intervention of latent infections is medically challenging. Since these viruses are species-specific, methods to study immune control are limited. The closely related murine -herpesvirus 68 provides a vital, small animal model system to dissect virus-host interactions that determine the pathogenic outcome in a natural host. The specific aims of the proposal are designed to evaluate the protective role of ISG54 both in an animal model of infection and in tissue culture systems, and to discover the molecular mechanisms of ISG54 that trigger apoptosis. Specific Aim 1: Establish the role of ISG54 at the host-pathogen interface. Murine models of disease have been invaluable to understand the role of the innate immune system in controlling pathogens. The ISG54 knockout mouse will be used to determine the mechanisms by which ISG54 regulates latent infection of the MHV-68 model pathogen. Specific Aim 2: Determine the molecular mechanisms of ISG54/IFIT2 action. Expression of ISG54 promotes mitochondrial apoptosis, and it is known that ISG54 exists within the cell in large molecular complexes with other proteins. Studies are designed to provide mechanistic insight to the impact of ISG54 on pro-apoptotic factors and critical binding partners. An in vivo model of disease and in vitro systems will be used to dissect the molecular mechanisms of an IFN-induced host defense gene at the viral interface. Achievement of our objectives is expected to provide knowledge necessary to harness the action of ISG54 and fill the need for new therapeutic development.